bigint txids vs 'xid' type, new txid_recent(bigint) => xid
Ahem. Forgot to squash in a fixup commit. Updated patch of txid_status(bigint) attachd.
A related patch follows, adding a new txid_current_ifassigned(bigint) function as suggested by Jim Nasby. It's usefully connected to txid_status() and might as well be added at the same time.
Since it builds on the same history I've also attached an updated version of txid_recent(bigint) now called txid_convert_ifrecent(bigint), per the above-linked thread.
Finally, and not intended for commit, is a useful test function I wrote to cause extremely rapid xid wraparound, bundled up into a src/test/regress test case. txid_incinerate() can jump the server about UINT32/2 xids in ~2 seconds if fsync is off, making it handy for testing. Posting so others can use it for their own test needs later and because it's useful for testing these patches that touch on the xid epoch.
